1. Field of the Invention
The present invention relates to a device for protecting a coil of a brushless motor against thermal damage, and more particularly, to a device for preventing a motor coil from burning in case the motor rotation stops due to some cause.
2. Description of the Prior Art
In recent years, applications of small DC motors have been expanding at a very rapid pace not only in the audio industry but also in the information industry because of their excellent controllability. Among them, because of the elimination of mechanically contacting parts such as a brush or commutators and the advantage of longer life, brushless motors are finding increasing applications as industrial motors which require particularly high reliability.
Because of this, DC power has been replacing AC power as the driving source for small axial fans over the past few years, and DC axial fans using a brushless motor have come to be used in increasing numbers.
Also, as industrial apparatuses such as computer-controlled apparatuses have come to be constructed in a higher packaging density, there has been an increasing need for DC axial fans for forced-air ventilation use, along with increasing requirements for higher reliability of such fans in themselves. That is, in case a motor stops due to some cause, it is required not only that the motor coil be protected against thermal damage, but also that a possible accident be prevented by issuing some kind of warning to the control unit of the apparatus, while the apparatus should be automatically reset to restart the motor for proper rotation after the cause has been removed. Therefore, it is usual to provide such an apparatus with a device for preventing a coil of a motor from burning.
FIG. 3 shows a conventional device for preventing a coil of a DC axial fan motor from burning. If the motor stops due to an external cause or continues to run at a slow speed with overload, a motor coil 4 will burn out, and the motor will no longer function properly. In order to prevent such troubles, the device of FIG. 3 is constructed as described below to deactivate the output circuit 3 so that overcurrent will not flow through the coil 4.
The device of FIG. 3 comprises a magnetic flux detection unit 1, a position signal amplification circuit 2, an output circuit 3, and a rotation detection circuit 5. The magnetic flux detection unit 1 produces a position signal in accordance with the rotation of the rotor of the motor. The position signal is amplified by the position signal amplification circuit 2 to be supplied to both the output circuit 3 and the rotation detection circuit 5. When the line from the circuit 2 is HIGH, the output circuit 3 energizes the coil 4 of the motor. The rotation detection circuit 5 generates a rotation pulse signal synchronous with the motor rotation, in response to the signal fed from the position signal amplification circuit 2.
The device of FIG. 3 further comprises a constant-current circuit 46, a control circuit 47, a discharge circuit 48, and a capacitor 49. The constant-current circuit 46 is provided with a constant-current source 51, transistors 52 and 53, and a resistor 54, to constitute a current mirror circuit. The circuit 46 supplies a current f a constant value determined by the resistor 54 to the capacitor 49 which is connected to the collector of the transistor 53. The control circuit 47 is provided with resistors 55 and 56, a comparator 57, and a transistor 58 the collector of which is connected to an input 3a of the output circuit 3. The inverted input terminal of the comparator 57 is connected to V.sub.c. The discharge circuit 48 has resistors 59 and 60 a comparator 61, and transistors 50 and 62. The base of the transistor 62 is connected to the output of the rotation detection circuit 5. The inverted input terminal of the comparator 61 is connected to voltage V.sub.d. When the potential of the capacitor 49 rises to V.sub.c, the output of the comparator 57 becomes HIGH, and the transistor 58 is ON to make the input 3a of the output circuit 3 LOW, thereby deactivating the output circuit 3. When the rotor of the motor rotates at a normal speed, the rotation detection circuit 5 supplies periodically a detection signal (period: T.sub.1) to the transistor 62.
The operation of the device of FIG. 3 will be described with reference to FIG. 4 which illustrates the potential change of the capacitor 49. The transistor 62 is switched periodically by the rotation pulse signal to make the inverted input terminal LOW in synchronization with the rotation pulse signal, so that the capacitor 49 is periodically discharged by the transistor 50. In a normal rotation of the rotor, therefore, the potential of the capacitor 49 is prevented from rising up to the level V.sub.c (=V.sub.OFF) (curve A) so that rotor continues to turn.
On the other hand, when the motor stops due to an external cause, the rotation detection circuit 5 stops the generation of the rotation pulse signal, resulting in that the potential of the capacitor 49 continues to rise toward a point e. Upon the potential exceeding the point e, the output of the comparator 57 becomes HIGH to make the transistor 58 ON, by which the control circuit 47 is deactivated. When the potential further rises to reach a point f, i.e. V.sub.ON (=V.sub.d), the output of the comparator 61 becomes HIGH to make the transistor 50 ON, so that the capacitor 49 is discharged to reduce its potential to a zero level. Then, the transistor 58 of the control circuit 47 is turned OFF to reset the output circuit 3 to energize the coil 4. At this time, if the external cause has been removed, the motor returns to the normal state of rotation, but if not, the potential of the capacitor 49 rises again to reach a point h to deactivated the output circuit 3 once again (curve C).
In the operation mentioned above, the charging characteristic is determined by the capacitance of the capacitor 49 and the level of the constant current supplied from the collector of the transistor 53. It is generally required to set the period of one cycle (O-e-f-g) to a few seconds. The period of a few seconds may be obtained by (a) selecting the level of the constant current to be considerably low, by (b) making the capacity of the capacitor 49 large, or by (c) setting the value of V.sub.ON (or V.sub.d) to be high. The way (a) requires the value of the resistor 54 to be in the order of a few megohms. It is not possible to contain such a resistor in an IC. On the other hand, when the way (b) or (c) is adopted, the discharge transistor 50 must have a larger current capacity, which causes the production cost to increase.
FIG. 5 illustrates the potential change of a capacitor in an improved device. When the motor rotates normally, this device operates in the same manner as the device of FIG. 3, i.e., the potential change follows the curve A. When the motor stops due to an external cause, the potential of the capacitor rises to a point i at which the output circuit is deactivated, while the capacitor is discharged. Then, the potential of the capacitor drops to a point j to activate again the output circuit (curve D). The problem with this device is that, since the output circuit remains energized until the potential of the capacitor reaches the point i, a discharge current flows in a very short period of time when the capacitor is discharged with the external cause removed, which therefore requires a discharge transistor of a large current capacity.